1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a thin film used for an electronic component, a semiconductor integrated circuit, and the like.
2. Description of the Prior Art
At present, thin film formation techniques are indispensable to techniques of manufacturing electronic components and, particularly, semiconductor integrated circuits.
These techniques are more important for memory elements or memory integrated circuits using ferroelectrics or high dielectrics as a capacitor film material.
In a conventional Si-LSI process, a single-crystal Si substrate is used as a substrate. An Si oxide film and an Si nitride film are formed as ferroelectric films on the substrate, and a poly-Si film is stacked as an electrode film. These film formation techniques realize precise film thickness control and high-speed deposition.
At present, deposition techniques such as CVD and sputtering used in the Si-LSI process have been developed for ferroelectric and high-dielectric thin films which receive a great deal of attention as new capacitor dielectric films. Well-known solid materials exhibiting ferroelectric properties are composite metal compounds having a perovskite crystal structure and a layered perovskite crystal structure. As the composite metal compound having the perovskite crystal structure, lead zirconate titanate (PZT), BaTiO.sub.3, SrTiO.sub.3, and the like are known. As the composite metal compound having the layered perovskite crystal structure, SrBi.sub.2 Ta.sub.2 O.sub.9, PbBi.sub.2 Ta.sub.2 O.sub.9, and the like are known.
Many examples employing the above-described CVD and sputtering, and the sol-gel method as the deposition method have been reported. One example is disclosed in Japanese Unexamined Patent Publication No. 6-57411, and its technique will be described with reference to FIG. 1.
As shown in FIG. 1, a conductive film 123 is formed on a substrate 121 directly or through a buffer layer 122. A dielectric underlayer 124 is formed on the conductive film 123, and a perovskite oxide dielectric thin film 125 is formed on the dielectric underlayer 124. A metal film formed on the thin film 125 is processed to form an upper electrode 126.
C. K. Kwock et al. deposited a PZT thin film on a Pt underlayer by RF sputtering at a substrate temperature of about 200.degree. C. (Material. Research Society Symposium Proceed ings vol. 200. 1990. p. 83). After the PZT thin film was sputter-deposited on the Pt underlayer heated to a certain temperature, the resultant structure was annealed in a furnace to obtain perovskite crystals. In this case, it was reported that lead diffused outward from the film in high-temperature annealing, and the percent in lead loss from the film was proportional to the annealing temperature (see FIG. 2).
The present inventors also confirmed by experiments that constituent elements diffused into an underlayer even when SrBi.sub.2 Ta.sub.2 O.sub.9 (to be referred to as SBTO hereinafter) which has been receiving a great deal of consideration as a dielectric film material for a ferroelectric memory, was deposited. FIGS. 3A to 3D show the results of energy spectral analyses by energy dispersive X-ray spectroscopy (EDX) when an SBTO film was formed on a Pt/Ti film by magnetron RF sputtering without heating a substrate, and the resultant structure was annealed in oxygen at 800.degree. C.
In this case, the deposition film thickness was about 200 nm. The deposition underlayer was constituted such that a 500-nm thick Si oxide film was formed on an Si substrate, and a 20-nm thick Ti film and a 200-nm thick Pt film were formed on this Si oxide film.
Analyses were performed for an annealed structure and a structure not subjected to annealing. Analyzed portions were two portions, i.e., the SBTO film and underlayer (metal film) of each structure.
In deposition, no peak representing the presence of Bi element contained in the SBTO film was observed in the underlayer (metal film), as shown in FIG. 3B.
After annealing at 800.degree. C., the peak of the Bi element contained in the SBTO film appeared in the underlayer (metal film), as shown in FIG. 3D, which represented that the Bi element diffused from the SBTO film to the underlayer upon annealing.
In a film structure shown in FIG. 3A, a film immediately after deposition without heating a substrate consisted of crystals including many amorphous crystals or defects, so it did not exhibit ferroelectric properties or its properties were very poor. In FIG. 3C, crystallization was sufficiently performed upon heating to obtain desired ferroelectric properties. Therefore, the annealing process is necessary.
However, constituent elements contained in the dielectric thin film also diffuse during crystallization. Atoms which do not contribute to the crystal growth are precipitated in the film or diffuse outside it. "Elements which do not contribute to the crystal growth" means both "elements excessive in constituting crystals" and "elements which diffuse before being entrapped in crystals". When an SBTO film is used as a dielectric thin film on a Pt/Ti underlayer, it is observed that Bi greatly diffuses to the underlayer.
FIG. 4 shows an example of measuring ferroelectric properties obtained when an SBTO film is sputter-deposited on a Pt/Ti underlayer on an Si oxide film and annealed in the oxygen atmosphere at 800.degree. C., and then an upper Pt electrode is arranged to form a capacitor structure. The composition ratios of Bi to Sr before and after annealing which are examined by the ICP (Induced Coupled Plasma) analysis are 2.1 and 2.0, respectively, which are almost stoichiometric composition ratios.
The polarization characteristics, however, are almost 10% the reported value (2Pt=15 to 20 .mu.C/cm.sup.2), and are not preferable. This is because many Bi components diffuse at the interface region with the underlayer before being entrapped in crystals in crystal growth, resulting in Bi loss at the interface with the underlayer, or because Bi necessary for crystallization decreases upon diffusion of Bi to the underlayer, resulting in a condition with many defects or a region in a condition wherein a desired crystal structure does not satisfactorily grow.
As described above, in the method of depositing a composite metal oxide as a dielectric thin film, when the film is deposited without heating a substrate to a high temperature, the constituent elements of the dielectric thin film diffuse outward or into an underlayer. As a result, the composition ratio of the elements of the dielectric thin film crystallized by annealing undesirably changes. In particular, the distribution at the interface between the surface region of the film and the underlayer is disadvantageously greatly influenced.
When the film is deposited while heating the substrate, if an element having a high vapor pressure is contained as a constituent element in film formation, a crystalline thin film is difficult to grow, while ensuring the stoichiometric composition ratio. Especially, the composition disadvantageously shifts at the interface region with the underlayer. In addition, the constituent elements of crystals are lost to interfere with the film growth. Even if a dielectric thin film serving as a buffer layer and having dielectric properties and the like which hardly influence target properties is deposited as an intermediate layer on an underlayer on which the dielectric thin film having necessary properties should be deposited, the film quality is degraded during deposition unless the respective deposition temperatures are properly controlled.
In Japanese Unexamined Patent Publication No. 6-57411 shown in FIG. 1, as the dielectric underlayer 124 of the perovskite dielectric thin film 125 consisting of ABO.sub.3 (A and B are perovskite crystals at A and B sites as atom positions generally designated), a dielectric thin film consisting of A'B'O.sub.3 or B'O.sub.3 using the same constituent elements is effective.
If this underlayer is formed of a dielectric, the composition ratio of the elements of the formed film must exhibit dielectric properties. That is, when a dielectric layer is formed on the dielectric underlayer 124 so as to contact the dielectric underlayer 124 deposited with a precise composition as a dielectric, if the formation temperature is high, or if energy to promote generation of point defects is applied to promote diffusion of elements, diffusion of the elements of the underlayer to the lower conductive film 123 (lower electrode) is difficult to suppress, resulting in poor film properties.
In Japanese Unexamined Patent Publication No. 6-57411, the buffer layer 122 is arranged between the conductive film 123 and the substrate 121 to prevent mutual diffusion between the substrate and the dielectric film. Even with this arrangement, diffusion of the elements of the dielectric layer to the conductive film 123 is difficult to prevent.